It is generally impractical for a single VHF/UHF RF repeater transmitting site to effectively serve a large geographical area. The broadcast area of a single site is limited by several factors. The effective radiated power of the antenna is subject to legal and practical limits. In addition, natural and man-made topographical features, such as mountains and buildings, block signals from certain locations.
Multiple transmitting sites are necessary to provide RF communications to all locations within a given locality. Multiple transmitters may be needed to cover a rural community covering many square miles or a city having many buildings. FIG. 1 is a schematic diagram of a simplified multiple-site system having three radio repeater (transmitting) central sites S1, S2, and S3 providing communications to geographic areas A1, A2, and A3, respectively. Mobile or portable transceivers within area A1 receive signals transmitted by site S1, transceivers within area A2 receive signals transmitted by site S2, and transceivers within area A3 receive signals transmitted by site S3. Each site has a site controller that acts as a central point for communications in the site. To enable communications from one area to another, a switch networks the radio systems together to establish audio slots connecting one site controller to another. Thus, a caller in one area can communicate with someone in another area.
The present invention is directed to a multisite RF trunked repeater system. Mulitsite allows a caller in one area (e.g. A1) to communicate with a callee in another area (e.g. A2). Multicast broadcasts signals only into those areas where the intended callee(s) is located. Moreover, in a multicast network, each site assigns a specific channel to a call independently of the channel assignments made by other sites. Thus, a single call may be broadcast from several site transmitters each operating on a different frequency.
In multisite, the site controller (S1) receives a call from a mobile radio in A1 requesting a channel to communicate with a specific callee. A caller requests a channel simply by pressing the push-to-talk (PTT) button on his microphone. This informs the site controller that a channel is requested. The PTT signal is transmitted to the unit on a control channel that is continuously monitored by the site controller. The site controller assigns a channel to the call and instructs the caller's radio unit to switch from the control channel to the channel assigned to the call. This assigned channel is applicable only within the area covered by the site.
In addition, the site controller sends the channel assignment to the multisite network switch. The switch assigns an internal audio slot to the call. The switch also sends a channel request to all other site controllers or to only those site controllers having a designated callee within their area. Upon receiving a channel request, these secondary site controllers assign a channel to the call. Again, each secondary channel is operative only in the area covered by the secondary site controller. The secondary site controller(s) also sends the channel assignment back up to the multisite switch. The caller can then communicate with a unit or group in an other area via the multisite switch. The call is initially transmitted to the primary site controller, routed through the assigned audio slot in the switch and retransmitted by the secondary sites on various assigned channels in those other areas.
When the caller ends the call, the primary site controller deactivates the assigned channel for that site and notifies the network switch that the call is terminated. There may be a brief "hang time" after the end of the call during which the channel remains assigned. During this hang time, the call can be rekeyed without going through the channel assignment procedure.
When the call is dropped, the network switch sends an end of call command to the secondary site controllers. A call is terminated in a similar format and operation as the slot assignment. Instead of establishing an audio slot, the end of call command causes the assigned slots and channels to be released.
In addition to providing communications between mobile radio units in different areas, the multisite network switch provides communications between dispatchers in different areas and between dispatchers and mobile radio units in different areas. The dispatcher consoles are connected to the network switch in the same manner as are the site controllers. A dispatcher console can issue a channel call request through the network switch to a site controller in another area to call a mobile unit or to another dispatcher console to call a dispatcher in another area.
In addition to all of the features that the mobile units have, each dispatcher console has the ability to participate in any call in its area or to its assigned groups. Thus, when a call comes through the network switch from another area to a mobile radio, the network switch informs the dispatcher console of the call in addition to notifying the site controller. The dispatcher can listen in or participate in the call to the mobile radio.
The network switch is also capable of handling calls to groups of mobile units and/or dispatcher consoles. The wide area switch manages group calls and monitors the network to ensure that the site controllers for all of the callees in the group assign a channel to the group call. If a channel is not assigned, the wide area switch advises the caller that the wide area call cannot be formed as requested. The caller then has the option of re-keying the call so as to reach those areas having assigned channels.
The present invention relates to a multisite switch having a distributed architecture. The logical functions of the switch are shared by various microprocessor operated nodes distributed throughout the switch. The nodes share the computational workload of the switch. Each node is connected to a site controller, dispatcher console, the system manager or other component of the overall radio system. The nodes coupled to site controllers are referred to as Master II Interface Modules (MIMs) and the nodes coupled to dispatcher consoles are referred to as Console Interface Modules (CIMs).
The distributed architecture of the multisite switch safeguards against catastrophic failures of the switch or of all communications from one RF system to another. The multisite switch does not completely fail if one card breaks down. Wide area communications, i.e., calls between site areas, continue despite the failure of a node. If a card fails, then the gateway to the network is closed only for its site controller or dispatcher console. Failure of a node prevents wide area communications only with respect to the site or console connected to the failed node. Mobile units in the area serviced by the failed card will not be able to call a unit in another area or receive calls from another area.
Local communications within an area are not disabled by the failure in the multisite switch. A site controller is not disabled by a failure of its associated node in the multisite switch. In particular, the failure of a MIM does not disable the site controller to which the MIM is connected. The site controller continues to operate and local communications within the area are unaffected by a failure in the multisite switch.
The ability to continue wide area calls after a node in the switch has failed provides several advantages to a distributed architecture switch over a central architecture switch. In a central architecture, a central processing unit (CPU) governs the operation of the switch. If this CPU fails, then the entire switch fails. Wide area communications are completely shut down by the failure of a multisite switch having a central architecture. As already stated, wide area communications are not completely shut down by a failure in a switch having a distributed architecture.
Distributed network multisite systems have a much faster data transfer rate than comparable central architecture multisite systems. Central computers process information serially. All communications passing through the switch must be serially processed by the central computer. The central computer slows communications because of its serial operation. Distributed network systems achieve parallel processing by sharing the computational task between several processors. Distributed networks are generally significantly faster than central computers.
Distributed network multisite systems are generally less expensive than multisite systems having a central computer. The hardware needed for a distributed network is a series of microprocessor controlled cards that handles communications between the multisite switch and the site central controllers, dispatcher consoles and various other users of the network. The cost of a series of cards is typically much less than that of a central computer. Moreover, a distributed network switch can be expanded simply by adding cards. To expand the capacity of a central computer requires purchasing a larger central computer.
Each node of a multisite network switch is supported by a switch controller card and one or more audio cards. These node units all have the same architecture and are interchangeable. The same controller cards and audio cards are used in all nodes.
The node architecture is novel and provides advantages over prior architectures used in RF trunking systems. The multisite node architecture provides for an interchangeable node unit that can be inserted in any node in the switch. The multisite switch can be serviced in the field by replacing the node unit. The service person need only stock one type of node unit to replace any node in the switch. The service technician no longer must stock a variety of components to service the switch or review voluminous manuals about the circuity in each of the various nodes. Similarly, a uniform node architecture reduces the complexity and costs of manufacturing.
The architecture of the node is also novel in that a single controller board supports a plurality of audio boards that themselves handle several audio channels. In the preferred embodiment, one controller board supports eight audio boards that each carry four audio/data channels. Thus, one controller board supports thirty-two (32) audio/data channels. The architecture of the node and its operation are specifically designed to enable a single controller board to handle a large number of audio boards and channels.
For example, the operational load on the controller board was minimized by assigning audio bus/slots to each channel for audio transmissions when the multisite switch is first enabled. Accordingly, when an audio transmission is received on a channel from the site controller the linkage between the channel and an audio bus/slot in the switch already exists. The controller board is free to process the channel assignment and related switch messages. The controller board does not have to divert its processing capacity to establish the linkage between the channel and bus/slot.
In addition, the controller board architecture has two principal processing units. The first is an interface microprocessor that is responsible for most of the logical processing functions of the node and the other is a communications controller that routes command messages between the interface processor, the internal switch message bus and the external serial link to the RF site, dispatcher console or other RF unit. The controller card also has a dual-port RAM memory which conveys messages and other communications between the interface processor and the communications controller.
The communications controller is principally a message router. Once a valid message is received at the node, the communications controller loads the message into the dual-port RAM and signals the interface processor that it has a message waiting. The interface processor retrieves the message and processes it accordingly. Similarly, the interface processor generates messages for transmission to the site or dispatcher, or for broadcast on the switch message bus. The processor loads its message in the dual-port RAM and notifies the communications controller that a message is waiting. The communications controller retrieves the message and routes it to the message bus or to the external serial port depending upon the address of the message within the dual-port RAM. The dual-port RAM is segregated into memory buffer packets. Each packet is allocated for messages either to and from the message bus, the external serial port, and the interface processor.
This controller board architecture enables a single interface processor to handle the processing requirements for a node in the distributed multisite switch. The architecture insulates the interface processor so that it is reserved for message processing and is not required to perform message routing functions. These message routing functions are performed by the communications controller.
While the communications controller sends all types of messages to the interface processor via the dual-port RAM, the controller does not send redundant messages to the processor. A large portion of the message traffic on the switch message bus relates to the status of each of the audio bus/slots in the switch. One of the few processing functions of the communications controller is to discard all redundant bus/slot status messages. This function of processing bus/slot status messages is more fully described in application Ser. No. 07/658,640, filed Feb. 22, 1991, entitled message "Bus Slot Update/Idle Control In RF Trunking Multisite Switch" cited above. Accordingly, the novel architecture of the controller board enables the node to use its processing capability to handle the node's share of the processing load of the switch.
In addition to those described above, there are many other advantages of the multisite RF system over conventional RF trunked systems. Many of these advantages are apparent from the following detailed description of the invention.